How children up risk of a woman getting arthritis
A woman's children can increase her risk of rheumatoid arthritis, research suggests.
Foetal cells from an unborn baby may remain in a mother's body for decades and trigger an immune reaction linked to the disease, scientists believe.
But whether or not this happens depends on a child's genetic make-up.
US researcher Giovanna Cruz, from the University of California at Berkeley, said: "During pregnancy, you'll find a small number of foetal cells circulating around the mother's body, and it seems that in some women, they persist as long as several decades.
"Women with rheumatoid arthritis are more likely to have this persistence of foetal cells, known as foetal microchimerism, than women without the condition, suggesting that it is a potential risk factor for the development of rheumatoid arthritis.
"Why it happens, we don't know, but we suspect HLA genes and their activity may be involved."
HLA genes are closely involved in the immune system's response to infection and its rejection of foreign tissue.
They "flag up" invaders and unwelcome cells, marking them out for immune system attack.
Certain versions of the genes are associated with rheumatoid arthritis (RA), an autoimmune disease in which the body's defence systems assault its own joints.
Women are three times more likely to develop RA than men, with peak rates among those in their 40s and 50s.
The fact that women are so susceptible to the disease strongly indicates a connection with pregnancy, said Ms Cruz.
The researchers analysed HLA genes in women and their children. They found that having children with the high-risk variants - inherited from the father - increased the risk of RA even after accounting for differences between mothers' genes.
This showed that beyond a woman's own genetic risk of RA, she could be put at additional risk by carrying children with certain HLA variants.
The findings, presented at the American Society of Human Genetics' annual meeting in San Diego, could lead to new ways of assessing a woman's risk of RA, said Ms Cruz.